refactor(electrobun): simplify bun backend — extract db-utils, merge handlers

- db-utils.ts: shared openDb() (WAL, busy_timeout, foreign_keys, mkdirSync)
- 5 DB modules use openDb() instead of duplicated PRAGMA boilerplate
- bttask-db shares btmsg-db's Database handle (was duplicate connection)
- misc-handlers.ts: 14 inline handlers extracted from index.ts
- index.ts: 349→195 lines (only window controls remain inline)
- updater.ts: removed dead getLastKnownVersion()
- Net reduction: ~700 lines of duplicated boilerplate

ui-electrobun/src/bun/search-db.ts

@@ -8,8 +8,8 @@
 
 import { Database } from "bun:sqlite";
 import { homedir } from "os";
-import { mkdirSync } from "fs";
 import { join } from "path";
+import { openDb } from "./db-utils.ts";
 
 // ── Types ────────────────────────────────────────────────────────────────────
 
@@ -32,13 +32,7 @@ export class SearchDb {
   private db: Database;
 
   constructor(dbPath?: string) {
-    const path = dbPath ?? DB_PATH;
-    const dir = join(path, "..");
-    mkdirSync(dir, { recursive: true });
-
-    this.db = new Database(path);
-    this.db.run("PRAGMA journal_mode = WAL");
-    this.db.run("PRAGMA busy_timeout = 2000");
+    this.db = openDb(dbPath ?? DB_PATH, { busyTimeout: 2000 });
     this.createTables();
   }